1. Field of the Invention
In order to achieve the most accurate reproduction of phonograph records in a high fidelity system, it is important that the magnetic phono cartridge of the system be operated into a specific termination impedance. The fundamental resonance of magnetic phono cartridges is typically located in the uppermost octave of the audible frequency range. Frequency response aberrations resulting from this resonance can be minimized only by operating the cartridge into the proper termination impedance. Failure to properly terminate a cartridge results in frequency response irregularities far larger than those tolerated in other parts of a high fidelity system. The audible consequences of improper cartridge termination can range from rather subtle to very significant, depending on the design of the cartridge and on the extent of mis-termination.
Virtually every manufacturer of magnetic cartridges specifies the ideal termination impedance for each of his products. This impedance consists of a specific resistance in parallel with a specific capacitance. In general, the equivalent capacitance of a given phono preamplifier will be appropriate to only a few cartridges. Additionally, the tone arm cable which is used to connect the cartridge to the phono preamplifier adds its capacitance to the input impedance of the preamplifier. Because of the variations in proper termination impedance among different cartridges, and because of the different input impedances presented by various preamplifier/tone arm cable combinations, it is no surprise that specific cartridges sound best with specific preamps since in the past proper termination has been achieved mainly by happenstance. In fact, the input impedance of some phono preamplifiers is not even equivalent to a pure resistance in parallel with a pure capacitance.
2. Description of the Prior Art
Presently, proper cartridge termination is usually not achieved. In concept one could limit his choice of preamp to those with appropriate input characteristics, measure with laboratory instrumentation the capacitance of the tone arm cable, perform the necessary calculations, and add the appropriate capacitor and resistor in parallel with the preamp input. In practice this procedure is seldom followed because it is tedious and requires the availability of relatively expensive laboratory equipment. Recently, some manufacturers have included one or more switches which control the input resistance and/or capacitance of the phono preamplifier, thus providing some facility to tailor cartridge termination impedance. Such a switch, however, does not eliminate the need to measure, and take into account, the capacitance of the tone arm cable.
It is a primary object of the present invention to provide a means for determining the incremental termination capacitance presented to the cartridge by connecting cables, thereby facilitating proper termination.
It is a further object of the invention to provide a method of properly terminating any magnetic phono cartridge irrespective of the input impedance of the particular phono preamplifier being used.